Add weight averaging and storing methods in references utils (#3352)

* Adding the average_checkpoints() method.

* Adding the store_model_weights() method.

references/classification/utils.py

-from collections import defaultdict, deque
+from collections import defaultdict, deque, OrderedDict
+import copy
 import datetime
+import hashlib
 import time
 import torch
 import torch.distributed as dist
@@ -252,3 +254,126 @@ def init_distributed_mode(args):
     torch.distributed.init_process_group(backend=args.dist_backend, init_method=args.dist_url,
                                          world_size=args.world_size, rank=args.rank)
     setup_for_distributed(args.rank == 0)
+
+
+def average_checkpoints(inputs):
+    """Loads checkpoints from inputs and returns a model with averaged weights. Original implementation taken from:
+    https://github.com/pytorch/fairseq/blob/a48f235636557b8d3bc4922a6fa90f3a0fa57955/scripts/average_checkpoints.py#L16
+
+    Args:
+      inputs (List[str]): An iterable of string paths of checkpoints to load from.
+    Returns:
+      A dict of string keys mapping to various values. The 'model' key
+      from the returned dict should correspond to an OrderedDict mapping
+      string parameter names to torch Tensors.
+    """
+    params_dict = OrderedDict()
+    params_keys = None
+    new_state = None
+    num_models = len(inputs)
+    for fpath in inputs:
+        with open(fpath, "rb") as f:
+            state = torch.load(
+                f,
+                map_location=(
+                    lambda s, _: torch.serialization.default_restore_location(s, "cpu")
+                ),
+            )
+        # Copies over the settings from the first checkpoint
+        if new_state is None:
+            new_state = state
+        model_params = state["model"]
+        model_params_keys = list(model_params.keys())
+        if params_keys is None:
+            params_keys = model_params_keys
+        elif params_keys != model_params_keys:
+            raise KeyError(
+                "For checkpoint {}, expected list of params: {}, "
+                "but found: {}".format(f, params_keys, model_params_keys)
+            )
+        for k in params_keys:
+            p = model_params[k]
+            if isinstance(p, torch.HalfTensor):
+                p = p.float()
+            if k not in params_dict:
+                params_dict[k] = p.clone()
+                # NOTE: clone() is needed in case of p is a shared parameter
+            else:
+                params_dict[k] += p
+    averaged_params = OrderedDict()
+    for k, v in params_dict.items():
+        averaged_params[k] = v
+        if averaged_params[k].is_floating_point():
+            averaged_params[k].div_(num_models)
+        else:
+            averaged_params[k] //= num_models
+    new_state["model"] = averaged_params
+    return new_state
+
+
+def store_model_weights(model, checkpoint_path, checkpoint_key='model', strict=True):
+    """
+    This method can be used to prepare weights files for new models. It receives as
+    input a model architecture and a checkpoint from the training script and produces
+    a file with the weights ready for release.
+
+    Examples:
+        from torchvision import models as M
+
+        # Classification
+        model = M.mobilenet_v3_large(pretrained=False)
+        print(store_model_weights(model, './class.pth'))
+
+        # Quantized Classification
+        model = M.quantization.mobilenet_v3_large(pretrained=False, quantize=False)
+        model.fuse_model()
+        model.qconfig = torch.quantization.get_default_qat_qconfig('qnnpack')
+        _ = torch.quantization.prepare_qat(model, inplace=True)
+        print(store_model_weights(model, './qat.pth'))
+
+        # Object Detection
+        model = M.detection.fasterrcnn_mobilenet_v3_large_fpn(pretrained=False, pretrained_backbone=False)
+        print(store_model_weights(model, './obj.pth'))
+
+        # Segmentation
+        model = M.segmentation.deeplabv3_mobilenet_v3_large(pretrained=False, pretrained_backbone=False, aux_loss=True)
+        print(store_model_weights(model, './segm.pth', strict=False))
+
+    Args:
+        model (pytorch.nn.Module): The model on which the weights will be loaded for validation purposes.
+        checkpoint_path (str): The path of the checkpoint we will load.
+        checkpoint_key (str, optional): The key of the checkpoint where the model weights are stored.
+            Default: "model".
+        strict (bool): whether to strictly enforce that the keys
+            in :attr:`state_dict` match the keys returned by this module's
+            :meth:`~torch.nn.Module.state_dict` function. Default: ``True``
+
+    Returns:
+        output_path (str): The location where the weights are saved.
+    """
+    # Store the new model next to the checkpoint_path
+    checkpoint_path = os.path.abspath(checkpoint_path)
+    output_dir = os.path.dirname(checkpoint_path)
+
+    # Deep copy to avoid side-effects on the model object.
+    model = copy.deepcopy(model)
+    checkpoint = torch.load(checkpoint_path, map_location='cpu')
+
+    # Load the weights to the model to validate that everything works
+    # and remove unnecessary weights (such as auxiliaries, etc)
+    model.load_state_dict(checkpoint[checkpoint_key], strict=strict)
+
+    tmp_path = os.path.join(output_dir, str(model.__hash__()))
+    torch.save(model.state_dict(), tmp_path)
+
+    sha256_hash = hashlib.sha256()
+    with open(tmp_path, "rb") as f:
+        # Read and update hash string value in blocks of 4K
+        for byte_block in iter(lambda: f.read(4096), b""):
+            sha256_hash.update(byte_block)
+        hh = sha256_hash.hexdigest()
+
+    output_path = os.path.join(output_dir, "weights-" + str(hh[:8]) + ".pth")
+    os.replace(tmp_path, output_path)
+
+    return output_path